Sen. John McCain, R-Ariz., said that Congress must have "significant oversight" over any expansion of DIA's espionage operations. (Alex Wong / Getty Images)

A special congressional panel, worried about the disappearing lines between military and intelligence work, is endorsing Senate-approved language restricting plans to insert the U.S. military into the global espionage business.

At issue is a Pentagon plan to expand the size and scope its top intelligence arm, the Defense Intelligence Agency, by adding 1,600 covert operatives — also known as spies. For decades, America’s global espionage efforts have been carried out by the CIA. The current size of the DIA’s covert corps appears to be classified.

Asked on Dec. 18 by Defense News whether senior defense lawmakers adopted the restrictions over worries about a “slippery slope” on which the nation’s covert spycraft could eventually become overly militarized, Sen. John McCain, R-Ariz., replied sternly: “Absolutely.”

That is one reason why members of a House-Senate conference committee included the upper chamber’s language that would require the Pentagon to submit stacks of information about the proposed DIA changes before the plan could be fully implemented.

“We just think that it’s important that we have significant oversight of what they’re doing,” McCain said. “We’re not saying they shouldn’t do it. We’re not saying they can’t do it. We’re saying we want to make sure if there’s any increase that we’re fully informed. … More about the fact that if you’re going to expand ... a covert capability, we have oversight responsibilities.”

“It’s just that we know once their restricted, they’ll come over to see us,” McCain said, adding with a chuckle: “They’ll come to visit, I’ll promise you that!”

On its own website, the Pentagon’s top intel agency describes its current duties this way: “The Defense Intelligence Agency (DIA) is first in all-source defense intelligence to prevent strategic surprise and deliver a decision advantage to warfighters, defense planners, and policymakers. DIA deploys globally alongside warfighters and interagency partners to defend America’s national security interests.”

But under the expansion proposal — first briefed to some lawmakers earlier this year — these military clandestine operatives would conduct spycraft rather than just gather information to aid military operations.

And in a series of recent interviews with Defense News, lawmakers from both political parties have expressed skepticism, with even supporters saying the Pentagon needs to proceed slowly while keeping Congress informed every step of the way.

“There has to be coordination,” Sen. Jack Reed, D-R.I. said. “What we [in Congress] have to do is make sure it is coordinated in a way not just within the Department of Defense, but with the intelligence community, as well.”

Pentagon and Obama administration officials must first, before lawmakers approve the plan, spell out “the trade-offs, in terms of the opportunity costs and also the benefits,” Reed told Defense News. “The burden of proof is on them to say, ‘This is more cost-effective, this is critical to our national security’.”

That is the kind of information sought by the Senate-backed language adopted by a House-Senate panel that on Dec. 18 agreed to a final version of 2013 Pentagon policy legislation.

The provisions would bar the Defense Department from spending funds on clandestine personnel above the number the military employed April 20.

They would also mandate the Pentagon provide Congress an “estimate of the costs over the period of the current future-years defense program” — which spans 2013-2017 — “and an estimate of the out year costs,” states a report accompanying the upper chamber’s bill.

The Senate’s provisions also would require a report spelling out “a detailed description of the location and schedule for current and anticipated deployments of case officers … whether overseas or domestically.”

Further, the language would require DoD brass to clearly show how the DIA and the armed services would meet DoD goals for the new military clandestine services.

The House is expected to vote on and pass the 2013 Pentagon policy bill Dec. 20; Senate passage is expected shortly thereafter. While the White House has issued a veto threat over terrorist detainees provisions, Levin said he believes the conference committee approved language that would appease the White House’s concerns.